A Power Split Transmission (PST) (also known as an Electric, Variable Transmission (EVT)) is one type of transmission used for Hybrid Electric Vehicles (HEVs). Power split transmissions utilize two electric motors (“E-motors”) and require completely new transmission designs over conventional transmissions and existing HEV transmissions with a single E-motor. Power split transmissions, utilize epicyclic differential gearing (i.e., planetary gear sets) in a way that a part of the power of an internal combustion engine is transmitted through a mechanical path to the output, and another part is transmitted on an electrical path. The electrical path includes two E-motors, one of them operating as a motor and the other as a generator. Variable transmission ratios can be obtained by varying the speeds of the E-motors.
Typically, the PST allows no carryover of parts from conventional transmissions resulting in large development costs. However, HEVs with PST transmissions provide improved fuel economy over traditional series and parallel hybrids because the E-motors not only boost or regenerate during braking, but also drive in variable ratios. Those operations are done in EVT modes. Conventional geared transmissions have a certain number of gears (e.g., four to seven) with fixed ratios. Variable ratios in the EVT modes allow the engine to operate near the best efficient load point. In existing HEV PSTs, high power E-motors and power electronics are required in order to cover full load engine operation. In existing HEV PSTs, not all engine loads and speeds can be covered by the EVT modes. Further, existing HEV PSTs require high E-motor power, lack reverse torque, and include a small number of fixed gears.